There are many information processing apparatuses comprising a CPU, such as a microprocessor, that use a DRAM (Dynamic Random Access Memory) for saving of data for executing an OS, various applications, or the like, for temporary storage of data for executing image processing, or the like. The DRAM is connected to a CPU, an SOC (System On a Chip), or the like, and used. Also, in recent years, accompanying an increase in the number of functions and an increase in the sophistication of functions of information processing apparatuses, a memory bandwidth of DRAM is increasing, and in order to increase the memory bandwidth, a frequency of a clock for when a memory is accessed is made to be higher in standards such as DDR (Double-Data-Rate) 3, DDR4, or the like. In addition, the memory bandwidth is ensured by arranging a plurality of DRAM channels connected to a CPU, an ASIC (Application Specific Integrated Circuit), or the like. However, when an increase in the frequency of a clock, or when a plurality of memory channels are employed, a new problem of electric power consumption increasing occurs.
So, WideIO, which is a next generation DRAM standard, has currently been receiving attention. Regarding WideIO, a 3D stacking technique with a TSV (Through Silicon Via) is used, and configuration is taken layering DRAM chips on an SOC die. A characteristic of WideIO is that a high bandwidth of a maximum of 12.8 GB/second or more can be obtained with a 512 bit of wide data width, and also the WideIO has low-power consumption because an access frequency is kept low. Also, by employing the TSV, a package size can be made to be thinner and smaller compared to a conventional PoP (Package on Package). Furthermore, as a counter-measure to heat due to stacking memory in the SOC package, a temperature sensor, for detecting a temperature of the memory, is built in, and a self-refresh rate is changed in accordance with the detected temperature. Also, configuration may be taken so that a data width of 512 bits is divided into four channels of 128 bits each, and each channel can be controlled independently. For example, usage in which a channel 1 and a channel 2 are put into a self-refresh state, and a channel 3 and a channel 4 are used for normal memory access is possible. A basic configuration, a basic access approach, and the like, for WideIO are recited in US Patent Application Publication No. 2012/0018885 A1.
For DRAM, storage of data is performed by accumulating an electric charge in a capacitor equipped in each cell, and because these capacitors discharge due to a leak current of a semiconductor, it is necessary to charge the capacitor by a refresh operation in order to hold the data in the DRAM. Because this discharge of electric charge depends on the temperature, and the higher the temperature, the faster the discharge speed becomes, it is necessary to enhance the refresh rate when the temperature of the DRAM becomes higher.
In conventional memory access control approaches, the auto-refresh rate set for a memory controller is set in accordance with a maximum temperature that the DRAM reaches. For this reason, in a case where the temperature of the DRAM is low, there is room to set the auto-refresh rate to be lower, and suppress electric power consumption to improve access performance.
In order to decide an appropriate auto-refresh rate, the temperature of a position (hereinafter referred to as a hotspot) where the temperature has become highest on the DRAM chip may be detected in real-time, and a sufficient auto-refresh rate for that temperature may be set. However, there are the following issues.
The first issue is being able to obtain the temperature of the hotspot precisely. Because hotspots in a stacked DRAM chip occur more markedly than in conventional type chips, it is known that temperature differences between a hotspot and a non-hotspot become large. Thus, in a case where the previously described temperature sensor, for example, is positioned in a non-hotspot, a large discrepancy between the temperature that the sensor detects and the temperature of the hotspot occurs. As a result, a correction of the auto-refresh rate based on the temperature measured by a temperature sensor is not performed correctly, and this leads to an increase in electric power consumption, a disappearance of content stored in the DRAM, or the like.
A second issue is that processing for detecting the temperature of a hotspot in real-time and setting the auto-refresh rate corresponding to the detected temperature becomes overhead.